I had Marathons before but never rode them on a tandem or the GAP. I have done the gap several times on Schwalbe G-one Allrounds at 37c without issue (loaded, with me at 200# and a 29# bike). Unless it's wet, the gap surface is very good, not very loose, so while more of a road tire I wouldn't expect much of an issue with your Marathons, but you have the experience of how they feel loaded off-pavement.